Cement and Concrete Research 30 (2000) 543 ± 551Mechanical treatment of fly ashes Part IV. Strength development of ground fly ash-cement mortars cured at different temperatures  *, J. Monzo  , M.V. Borrachero, E. Peris-Mora, F. Amahjour J. Paya  n en Quõ Âmica de los Materiales de Construccio  n (GIQUIMA), Departamento de Ingenierõ Âa de la Construccio  n, Grupo de Investigacio  cnica de Valencia, Camino de Vera s/n 46071, Valencia, Spain Universidad Polite Received 5 January 1999; accepted 20 December 1999 Abstract The influence of fly ash grinding on the strength development of fly ash/cement mortars cured at different temperatures was studied. A significant increase of compressive strength (Rc) for fly ash mortars was found at early age (3 days) when curing temperature is raised. However, the highest Rc values at 28 days' curing time were obtained for fly ash mortars cured at 40°C. The higher reactivity of fly ashes was found for ground fly ashes (GFA) compared to unground fly ash (T0). When Rc vs. curing time (in logarithmic scale) is represented, the slope values from regression analysis data obtained for the 20°C curing temperature series and origin ordinate from regression analysis data obtained for the 40°C or 60°C curing temperature series become good parameters for evaluating pozzolanic activity. An equivalence among Rc values obtained at curing temperatures of 20°C and 40°C and different curing times was proposed from strength gain data for mortars containing 15%, 30%, 45% and 60% replacing percentages. Finally, the compressive (Rc) and flexural (Rf) strength values for early ages and different curing temperatures were measured. A mathematical model has been proposed for the mechanical properties at early age of mortars containing fly ashes in 15 ± 60% replacement range and cured in 20 ± 80°C range. D 2000 Elsevier Science Ltd. All rights reserved. Keywords: Curing; Grinding; Thermal treatment; Compressive strength; Fly ash 1. Introduction It is well-known that fly ash concrete shows important strength gain as a consequence of increasing the curing temperature, whereas cement Portland concrete shows an increase in strength at early ages but a significant decrease is observed for mature concrete [1,2]. On the other hand, several upgrading procedures for fly ashes have been reported [3 ± 12]. However, very few studies about the influence of processed fly ashes on the mechanical strength of mortar or concrete cured at different temperatures have been carried out [12,13]. Obviously, the pozzolanic reaction rate of fly ashes towards hydrated lime increases with temperature [14] and kinetics of this reaction is influenced by curing temperature [15]. The mechanical properties of fly ash concrete depend on several interesting parameters, for example, the microstructure of the hydrated products [16] and the nature of colloidal and crystalline products formed in various conditions (curing time, relative humidity, curing temperature, etc.). Moreover, the chemical, physical, morphological and mineralogical parameters of fly ashes have a decisive influence on pozzolanic reaction, microstructure and nature of the hydrated products. The scope of this investigation was to conduct an indepth analysis of the influence of fly ash grinding on the strength of fly ash-cement mortars cured at different temperatures, as part of a complete study on the use of ground fly ashes (GFA) in concrete [17 ±19]. 2. Experimental A low-calcium fly ash (T0) was used for preparing GFA samples (T10, T40 and T60, corresponding to 10, 40 and 60 min of grinding time). The grinding procedure, physical * Corresponding author. Tel.: +34-6-387-7564; fax: +34-6-387-7564.  ). E-mail address:
[email protected] (J. Paya 0008-8846/00/$ ± see front matter D 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 8 - 8 8 4 6 ( 0 0 ) 0 0 2 1 8 - 0 65 7 Days 37.94 39.305 25.988 13.75 36. Mortars were prepared by mixing 450 g of this cement.44 specific gravity.98 48.990 0. water and binder in a 3:0.42 23.319 3.00 T (°C) 40 40 40 40 40 80 80 80 80 80 Binder C C/T0 C/T10 C/T40 C/T60 C C/T0 C/T10 C/T40 C/T60 3 Days 31.339 35.755 35.54% SO3. 2.54 34.25 37. immersed in water at a given temperature.544  et al.31 18.344 23. 1350 g of fine aggregate (natural sand.87% CaO.65 39. Mortars containing fly ashes were prepared replacing in weight 15%. 2.983 0.35 38.17 48. 14 and 28 days T (°C) 20 20 20 20 20 60 60 60 60 60 Binder C C/T0 C/T10 C/T40 C/T60 C C/T0 C/T10 C/T40 C/T60 3 Days 28.55 42.06 41.41 37.136 17.873 25. specimens were stored under water at experience temperature until test age. 7.110 0.20 33.083 b 15.06 40.661 22. 3.254 14.16 41. 4. Mortars containing a 30:70 fly ash/cement ratio were cured at 20°C. Table 1 summarizes the compressive strength data for this series.91 30.324 26.520 8.91 17.25 28 Days 46.620 R 0.44% loss on ignition.93 modulus fineness) and 200 ml of water.575 15.423 33.961 18. 0. 40°C. 1. A series of specimens were prepared for each fly ash type: original fly ash (T0) and GFA (T10. Mortar mixtures were put in a mold for obtaining three 16  4  4 cm3 specimens.1% MgO.4% SiO2.57 26.48 51.993 0.26 36.388 0.67 33.19 18.17 47.02 38.09 15.14 38.03 34.1% Na2O. 1. T40 and T60).24 51.710 0. 0.18].403 6.989 0. (1) for fly ash/cement mortars with 30% replacement percentage T (°C) 20 20 20 20 20 60 60 60 60 60 Binder C C/T0 C/T10 C/T40 C/T60 C C/T0 C/T10 C/T40 C/T60 % ± 30 30 30 30 ± 30 30 30 30 a 19. / Cement and Concrete Research 30 (2000) 543± 551 J.981 0.162 33.71 37. in order to study the different hydration kinetics and their consequence on strength development.36 7 Days 35. 16.720 20.218 4.074 29. 2.914 5. which were stored in a moisture room (20 1°C) for 24 h. 60°C and 80°C for 3.162 23. 6.710 0. which consists of ASTM Type I Portland cement blended with finely ground limestone (13% by mass).53 32.50 51.956 0.74 14 Days 43.79 38.52 41.934 0.5:1 ratio. 62. which presented the following chemical composition: 20.926 0. Flexural procedure was a center point load and then the two portions were tested in compression. 3.94% Al2O3.16 33.06 35. 60°C and 80°C.235 6.25 34.05 46. 45 and 60% of cement by original (T0) or GFA (T10.807 T (°C) 40 40 40 40 40 80 80 80 80 80 Binder C C/T0 C/T10 C/T40 C/T60 C C/T0 C/T10 C/T40 C/T60 % ± 30 30 30 30 ± 30 30 30 30 a 24.90 36. 30%.18 33. Paya Table 1 Compressive strength values (Rc in MPa) for 30:70 fly ash/cement mortars cured at 20°C.994 0. 26.95 41.12 36. 40°C. Results and discussion A first batch of specimens were prepared as follows: mortars were composed of sand.85% Fe2O3.59 35.13 34.02% loss on ignition and 0.63 52.95% insoluble residue.1% CaO.30 31.125 28. the compressive strength values of control mortar and fly ash mortars increase progressively with curing time.15 34.54 43.65 38. Prismatic specimens (160  40  40 mm) were cured for the first 24 h at 20°C and. 7.50 26.937 0.81 54. 0.703 21.05% MgO.69 25.69 37.49 29. The cement for preparing pastes was an ASTM type I Portland cement.862 32.21 25.13 31.344 5.57 35.907 0.2% Al2O3.24 36.214 2.367 24. after demoulding. being binder cement or a mixture of cement and fly ash.56 24. T40 or T60) fly ashes.830 .357 R 0. 2810 cm2/g Blaine fineness and 2.92 properties and mineralogical and chemical compositions of fly ashes have already been reported [17.65 41. The cement used for preparing mortars [12] was commercial Spanish cement II-F/35A.02 39.390 2.0% Fe2O3.95% K2O.69 37. 5% K2O.14 14 Days 38.672 0. in general. after demoulding. 14 and 28 days age. The original fly ash (T0) presented the following chemical and physical parameters: 41.10% Na2O.32 49.802 26. As expected.21% SiO2. 60°C and 80°C and compressive and flexural strength were tested at 3.486 6. 0. 3.95 37.990 0.17 32.79 34.590 4.84 38.87 28 Days 46.58 35.33 41.36 21.95 30. The selected curing temperatures were 20°C. 40°C.602 31.777 22.570 b 17.993 0.81 28. In order to establish good correlation between compressive strength and curing time Table 2 Analysis regression parameters for linear correlations according to Eq.994 0. In this manner. / Cement and Concrete Research 30 (2000) 543± 551 J. and a and b are constants for a given fly ash type and curing temperature. whereas the increase is less important when curing temperature is raised to 60°C and 80°C. respectively. From the analysis regression data for 30% fly ash replacement mortars. a very important increase of compressive strength with curing time for 20°C and 40°C can be observed. For the 40°C and 60°C data. In these figures. Compressive strength/curing time linear regressions for fly ash/ cement mortars with 30% replacement for different curing temperatures: (a) T0 fly ash. Fig. with a 30% replacing percentage. So. the compressive strength of fly ash mortars at an early age (3 days) increases with curing temperature. although differences between mortars containing T40 and T60 fly ashes were not much significant. 2a). consequently. 1a and b plots the compressive strength/curing time linear regressions for T0 and T60 fly ash mortars. and. curing temperature (Fig. The slope values for 80°C are very . (b) origin ordinate of regression line. two interesting observations can be made. (1) and curing temperature for cement/fly ash mortars: (a) slope of regression line. when the slope of these regression lines are plotted vs. Moreover. greater Rc values were obtained when fly Fig. increase in strength is limited. it may be noted that mortars made with GFA showed higher Rc values than mortars that contained original fly ash (T0). Paya 545 for the curing period of 3± 28 days. whereas the highest compressive strength values were found for 40°C at the age of 7. (b) T60 fly ash. values tend to change over: this observation suggests that the increase of curing temperature promotes the pozzolanic activity of fly ashes and high compressive strength values are obtained at very early age. When the Rc values of mortars containing fly ashes are compared (see Fig. 1. Correlation between regression data from Eq. 2. 14 and 28 days. First. t is curing time in days. 1a and b). et al. the slope of the linear regression for fly ash mortars cured at 20°C becomes a good parameter for evaluating pozzolanic activity. ash grinding time was increased. The linear regression data for prepared series of specimens are given in Table 2. the linear relationships were calculated [19] as follows: Rc a b log10 t
1 where Rc is the compressive strength in MPa. specially for highest reactive fly ashes. Fig. a good correlation between fly ash reactivity and the value of the slope at 20°C was found: T60 GFA presented the highest compressive strength and T0 the lowest one. 85 .15 11.50 30.22 46.70 28 Days 40. At 20°C. In low due to the rapid development of strength and no conclusion can be established.19]: wc SGi Ri À Ro
2 wc wfa where Ri is the compressive strength for a given replacing percentage and curing time. prepared with 15:85.546  et al.20 14 Days 35. However.37 33. the origin ordinate values were similar for all tested fly ashes. these differences on the reactivity of fly ashes are related mainly with fineness.74 33.97 37.89 16.59 16.61 29.33 31.02 19.30 7.00 9.20 16. the ordinate origin values for experiences carried out at 40°C (Fig.29 45. 14 and 28 days T (°C) 20 20 20 20 20 20 20 20 20 20 20 20 Fly ash (%) T0 (15) T0 (45) T0 (60) T10 (15) T10 (45) T10 (60) T40 (15) T40 (45) T40 (60) T60 (15) T60 (45) T60 (60) 3 Days 20. 4 shows the SG values for T0 and T60 fly ash containing mortars cured at 20°C and 40°C.56 47.02 15. (b) 60°C.63 36.61 16.08 26.53 24. 3a) and 60°C (Fig. A linear relationship has been obtained when specific surface area [19] was plotted vs.29 27.20 49. this value increases with temperature.52 25. The influence of the fly ash replacement level would be better studied by means of calculating the strength gain (SGi).24 7.65 31.58 10.41 42.40 10.75 50. 45:55 and 60:40 fly ash/cement ratios were cured at 20°C and 40°C.98 30. when the origin ordinate of these regression lines are plotted vs.16 30.38 28 Days 46. respectively.48 17. Blaine method.49 27.69 24.59 50. and. Ro the compressive strength for control mortar at the same age.83 16.21 38.36 T (°C) 40 40 40 40 40 40 40 40 40 40 40 40 Fly ash (%) T0 (15) T0 (45) T0 (60) T10 (15) T10 (45) T10 (60) T40 (15) T40 (45) T40 (60) T60 (15) T60 (45) T60 (60) 3 Days 29.72 9.65 41.31 12.60 33.94 21. The behaviour of specimens cured at 20°C was already reported [19].90 21.55 27. obviously.41 15.09 11. These correlations have been established for specific surface area values obtained from granulometric data (Sm) and for Blaine method (Sb).48 43.90 11.00 31.03 34. in order to obtain information about the behaviour of mortars cured at different temperatures and containing different fly ash/cement ratios (see Table 3 for compressive strength data).78 46.30 6.56 44.12 42.84 12.60 7. 3.70 7 Days 40.99 41. Paya Fig.30 23. Correlations between specific surface area values (Sb. Sm. and wc and wfa are the weight of cement and the weight of fly ash. In second place.22 48.42 26. Fig. additional information about the reactivity of fly ashes may be obtained. indicating that the addition acts as an inert material. at 40°C and 60°C Ð where fly ash pozzolanity is achieved Ð a good correlation between origin ordinate and type of fly ash can be proposed: T0 fly ash shows lowest pozzolanic activity.30 6.48 51. curing temperature (Fig.96 22. Linear regression data for this second batch of specimens are listed in Table 4. Compressive strength values fit to Eq.20 48. 3.66 7 Days 30. obtained from granulometric data) and origin ordinate values from the regression data for fly ash/cement mortars cured at: (a) 40°C. / Cement and Concrete Research 30 (2000) 543± 551 J.98 47.21 37.01 27. Obviously.12 42.68 37. A second batch of mortar specimens. 2b).54 21.04 41. 45:55 and 60:40 fly ash/cement mortars cured at 20°C and 40°C for 3.39 14 Days 45.19 34.25 19. according to the already reported Eq. a fact that can be seen in Fig.85 30.02 23. (2) [13.48 50. Origin ordinate should be assumed as the Table 3 Compressive strength values (Rc in MPa) for 15:85.18 21.02 31.30 33. 3b).53 11. and reactivity increases with grinding.01 20. (1) linear relationship.00 8.99 44. extrapolated compressive strength value for 1 day. It can be denoted that the increase in curing temperature yielded a pronounced increase in compressive strength for all replacing percentages.35 40.10 11.00 31.00 17. 7. 638 À0.985 0.948 0.432 b 20.903 10. 7 days at 40°C 60 ± 180 days at 20°C.916 15. Finally.009 23.326 0. (b) T60 fly ash.053 19.482 11.944 0.974 0.956 0. 45% and 60%) and.988 0.942 À0.069 7.970 0.943 0.578 13. . 40°C.992 0. 4.991 0.144 R 0. compressive strength values obtained for the 40°C experience presented approximately the following agreement with the corresponding compressive strength values for 20°C experience: 3 days at 40°C 14 ± 90 days at 20°C. / Cement and Concrete Research 30 (2000) 543± 551 J.957 0.975 0.783 24. 45% and 60% replacement percentages T (°C) 20 20 20 20 20 20 20 20 20 20 20 20 Binder C/T0 C/T0 C/T0 C/T10 C/T10 C/T10 C/T40 C/T40 C/T40 C/T60 C/T60 C/T60 % 15 45 60 15 45 60 15 45 60 15 45 60 a 11. Compressive strength gain (SGi) for fly ash mortars cured at 20°C and 40°C and different curing times: (a) T0 fly ash.529 18.600 21.124 24. and T60 became clearly much more reactive than T0 fly ash for all replacements studied.546 23.020 19.410 À1.742 À0.986 0.772 24.044 R 0. In this manner. as reported [19].971 T (°C) 40 40 40 40 40 40 40 40 40 40 40 40 Binder C/T0 C/T0 C/T0 C/T10 C/T10 C/T10 C/T40 C/T40 C/T40 C/T60 C/T60 C/T60 % 15 45 60 15 45 60 15 45 60 15 45 60 a 22.896 18.527 À1.400 23. prismatic specimens were prepared for several replacing percentages (15%.980 0. Again. The compressive and flexural strengths at this age were Fig.024 16.486 22.989 general.973 0.501 1.776 19. a third batch of specimens was performed in order to study the early-age compressive and flexural strength developments of cement mortars containing fly ashes.992 20. 14 days at 40°C 90 ± 180 days at 20°C. the influence of fly ash grinding and the influence of curing temperature.986 0.995 0.061 23.983 0. Paya 547 Table 4 Analysis regression parameters for linear correlations according to Eq.994 0.072 22.000 11.439 13.315 21.967 0. 60°C and 80°C) for 48 h. they were submerged in water at different temperatures (20°C.081 À2. after storing at 20°C for 24 h. et al.854 20. (1) for fly ash/cement mortars with 15%.982 3.981 0.865 20.955 23.977 0. 28 days at 40°C 180 ± 365 days at 20°C.445 19.187 21. mortars containing 30% and 45% replacing percentages yielded optimum SG values.768 11. 30%.435 22.203 11.243 À1.633 b 17.884 7. 55 40.00278 s 2. Since the study concentrates on the dependence of compressive strength with curing temperature of fly ash mortars at early ages. 5.7 . nearly all of T40 and T60 fly ash mortars cured between 40°C and 80°C and containing 15 ± 45% replacing percentage yielded compressive strength greater than 30 MPa.98 42.396 À0. c.548  et al. the highest compressive strength values are found for higher curing temperatures (60 ± 80°C) and lower replacing percentages (15 ± 30%). The contour graphical plotting of compressive strength values with fly ash replacing percentage and curing temperature are showed in Fig.5 3.00866 d 68.383 c À0. (3) for all tested fly ashes Fly ash T0 T10 T40 T60 a 39. 60°C curing temperature showed the best compressive strength values for all replacing percentages.00567 À0.403 À0. in general. T40 and T60 GFAs. 5 for all studied fly ashes. Thus. whereas an important part of T0 fly ash mortars for the same curing temperature range and replacing percentage range yielded compressive strength values of less than 30 MPa.00768 À0.00219 0. It can be observed that. Contour graphical plots for compressive strength of fly ash mortars cured at 20°C for 24 h and submerged in water at different temperatures (20°C to 80°C) for 48 h. a few alternative equations Table 5 Coefficients (a.87 62. Paya Fig. b.3 2. while T40 and T60 fly ash containing mortars yielded the highest rates.80 40.419 À0.0 2. Apparently. d and e) and standard deviation (s) of Eq.00727 À0.07 b À0.85 e 0. measured for mortars prepared using T0 and T10.00302 0.41 62.00169 0. / Cement and Concrete Research 30 (2000) 543± 551 J.46 61. Compressive strength values from Eq. T is curing tempera- ture (20 ± 80°C range). Finally. . j is replacing percentage (15 ± 60% range). Contour graphical plots for flexural strength of fly ash mortars cured at 20°C for 24 h and submerged in water at different temperatures (20°C to 80°C) for 48 h. 6. 7. et al. and a. (3) and coefficient values for T0 and T60 fly ash mortars. the compressive strength dependence with curing temperatures has been obtained for all studied replacing percentages. (3) for: (a) T0 fly ash. 6a Fig. d and e are the coefficients obtained by regression analysis from experimental data (see Table 5). c. were tested for regression analysis. b. as can be seen in Fig. the following mathematical model with the minimal values of s was selected: Rc a b j c
T À d 2 e jT
3 where Rc is the compressive strength (in MPa). From Eq. Paya 549 Fig. / Cement and Concrete Research 30 (2000) 543± 551 J. (b) T60 fly ash mortars. Peris Mora.000884 0.60 60. For 20°C and 40°C curing temperatures.R. 6a). the optimum Rc values for T60 fly ash mortars (Fig. 219. Two aspects may be noted: first. Peris Mora. 41 separacio (1991) 29. [2] T.475 5.J. d.579 5.00226 g À0. Monzo granulometric influence on mortar strength. Second. in: J. Department of Civil Engineering. 1982.J. Norway. Conclusions 1. Ramezanianpour. J.00125 À0. Schieûl. Trondheim. again.58 22. J.G. 2. However.A.00142 À0. g. the higher reactivity of GFA from pozzolanic activity point of view. Effectiveness of fly ash processing methods in improving concrete quality. Monzo  .78 3. an equivalence of the curing time for mortars cured at 20°C and 40°C has been established. The change of mortar properties as result of fly ash processing. Peris Mora. (3). P.  . Ottawa.'' which appeared in Eq. (4) for all tested fly ashes Fly ash a T0 T10 T40 T60 5.00115 d h e 0. On the other hand.). Ishii.560 49. Cem Concr Res 23 (1993) 917 ± 924. 6b) are reached at slightly lower temperatures than T0 fly ash ones (Fig. optimum values were obtained for 30% replacing percentage. suggesting the greater pozzolanic activity of T60 compared to other fly ash samples.63 16. ACI SP-114.92 51. 277. whereas the highest values of ordinate origin for the same type of mortars were found for series cured at 40°C and 60°C.00103 0. indicating. CANMET MSL 94-45 (IR).000682 s 0. By means of calculating the compressive strength gain (SG).912 b À0. References [1] V. In the same way. suggesting that the energy cost in extreme curing conditions (60 ±80°C) does not supply an important strength gain.114 4. 1994. Malhotra. 2nd edn. Conf. Improvements in the properties of concrete utilizing classified fly ash. The implications of a selected grade of United Kingdom pulverised fuel ash on engineering design and use in structural concrete. Proc 3rd CANMET/ACI Int.. Mater Constr (Madrid). J. p. 6. 95% of the value of optimum Rc is reached by curing the specimens in the range 50 ± 60°C. b. Sup. Good linear relationships were obtained between Rc values and the logarithm of curing time for fly ash/cement mortars cured at different temperatures. [8] S. higher curing temperature for yielding optimum Rc is required when fly ash replacing percentage increases. Slanicka. Norway. p. Fly Ash in Concrete. Influence of different sized frac[5] E. Optimum flexural strength (Rf) values were obtained for 30% fly ash replacing percentage. [7] P. 4. UK. finding highest Rf values for GFA mortars cured at 80°C.G. 3. After testing a few alternative equations. The curing temperature increase yielded an important increase in compressive strength for fly ash replacing percentages from 15% to 60%. 1989. in all cases. M.85 0. A preliminary study of fly ash [3] J. van der Sloot. A. Elsevier. Cem Concr Res 24 (1994) 791 ± 796.00138 À0. It can be noted that optimum compressive strength values are dependent on type of fly ash and on replacing percentage. Cem Concr Res 21 (1991) 285 ± 296. Th. 83.75 0. Paya  . e and h) and standard deviation (s) of Eq. 399. whereas T0 mortars showed highest Rf values cured at 60°C. 1.66 0. Williams. 8. Even so. Waste Mater Constr.00242 À0. Paya Table 6 Coefficients (a. 7. Goumans. this increase was lower for 60°C curing temperature. The dependence of flexural and compressive strength with curing time at early age for mortars containing different replacing percentages of fly ash has been studied.79 64. . Trondheim. Ha È rdtl. [9] R.J. whereas for GFA fly ashes. they are obtained at higher temperatures (70 ±80°C). Shigematsu. log curing time) became appropriate parameters for evaluating pozzolanic activity of GFAs and T0s. S. ACI SP-114. A. the flexural strength values Rf for this third batch of specimens were also measured. Ha È rdtl. the highest Rf values for T0 fly ash mortars were obtained at a curing temperature of about 60°C. p. Albers (Eds. Proc Intl Symp On the Use of PFA in Concrete. Cusens (Eds. Studies in Environmental Science 48.  . Two mathematical models have been proposed and the coefficients of the equations have been calculated for all tested fly ashes. 4. Vol. In this manner. 301.00238 À0.00128 0. Proc 3rd CANMET/ACI Int Conf. J. Paya  n balõ Âstica de una ceniza volante. p. and a parabolic term ``b(jÀh)2'' substitutes the linear term ``bj.). The calculated optimum compressive strength values for T60 fly ash mortars have been obtained for lower curing temperatures that T0 fly ash mortars. Eq. Ukita. 1. H. Monzo  . the mathematical model proposed for flexural strength calculation takes into account this second point. R. 1991.M. Thus. 7.29 21. In this case. 1989.550  et al. J. p. University of Leeds.00126 À0.72 and b. (4) was selected: Rf a b
j À h2 g
T À d2 ejT
4 The coefficients from regression analysis for fly ash mortars cured at different temperatures and with different replacing percentages are summarized in Table 6. Paya tions of a fly ash on workability of mortars. Owens. in: J. Estudios sobre cenizas volantes: [4] E. an important increase of compressive strength (Rc) values was observed in the 3± 28 days curing period.  . E. Papers. an evident increase in flexural strength is observed when fly ash is ground. 5. No significant increase was obtained for the highest tested temperature (80°C) due to the rapid development of Rc in these conditions. The influence of fly ash fineness on the strength of concrete.L. The highest slope value was found for mortars containing T60 fly ash cured at 20°C. Cabrera. [6] K. and contour graphical plots are showed in Fig.A. / Cement and Concrete Research 30 (2000) 543± 551 J. The slope and ordinate origin of these regression lines (Rc vs. 67. Acceleration of strength gain of lime ± Pozzolan cements by thermal activation. K. USA. E. Monzo  . p. J. M. Peris-Mora. Shi.L. C. Monzo  .  . V. Cem Concr Res 26 (1996) 225 ± 235. Proc 5th CANMET/ACI Intl Conf. Marchand. M. Cem Concr Res 27 (1997) 1861 ± 1874. R. Day. Peris-Mora. E. Cem Concr Res 23 (1993) 824 ± 832. E. Borrachero. Maltais. Hellewaard. Monzo  . Gonza  lezJ. J. J. Peris-Mora. M. Tercero. Bouzoubaa. Bilodeau. Early-strength development of Portland cement mortars containing air-classified fly ashes. R. Influence of curing temperature on cement 551 [16] [17] [18] [19] hydration and mechanical strength development on fly ash mortars. Mechanical treatJ. E. Development of microstructures in plain cement paste hydrated at different temperatures. Vissers. J. Detwiler. Monzo  . ACI SP-153. Paya  pez.J. et al.V.E. A. [12] J.L.V.E. Processed fly ash for high performance concrete. Cem Concr Res 21 (1991) 179 ± 189.M. M. Waste Manage. Physico-chemical characterization of ground fly ashes. Mechanical treatment of fly ashes: Part III. Kjellsen.V.V. Borrachero. Cem Concr Res 25 (1995) 1469 ± 1479. J. Borrachero. Gjùrv. Cem Concr Res 25 (1995) 449 ± 456. M. M. Cornelissen.A. Paya ment of fly ashes: Part I. 16 (1996) 119. Paya Pinillos. [11] N. Borrachero. Studies on strength Lo development of ground fly ashes (GFA)-cement mortars.O. O. Paya among magnetic and non-magnetic fly ash fractions: strength development of cement-fly ash mortars.  . Malhotra. The effect of grinding on the physical properties of fly ashes and a portland cement clinker. 1995. J. R. J. Milwaukee. Vol.H. Cem Concr Res 27 (1997) 1365 ± 1378. Mechanical treatment of fly ashes: Part II. Paya [10] H. WI.  . Gonza  lezJ. E. Borrachero. Paya  pez.J. Particle morpholoLo gies in ground fly ashes (GFA) and workability of GFA-cement mortars. Comparisons [13] J.V. .  . Peris-Mora. / Cement and Concrete Research 30 (2000) 543± 551 J.  . 1. Monzo  . Zhang. R. [14] C. Peris-Mora.W. E. [15] Y. Cem Concr Res 27 (1997) 1009 ± 1020. E.